Trioxane has been produced by cyclizing formaldehyde in its aqueous solution in the presence of an acidic catalyst such as sulfuric acid, cation exchange resin having a sulfonic acid group, zeolite or the like.
These conventional processes for trioxane production have not been satisfactory because of various problems such as corrosion of production equipment, precipitation of paraformaldehyde at high formaldehyde concentration, low trioxane selectivity and the like. For example, when a sulfuric acid catalyst is used, there is a problem of equipment corrosion by strong acid and it is necessary to use an expensive material(s) for equipment to prevent the problem. When a cation exchange resin having a sulfonic acid group is used as a catalyst, there are problems such as low trioxane selectivity, paraformaldehyde precipitation and consequent pressure loss of catalyst layer, and the like. Meanwhile, it is well known that increased methanol concentration in reaction system is effective for the prevention of paraformaldehyde precipitation in aqueous formaldehyde solution. Increased methanol concentration in reaction system, however, invites formation of larger amounts of by-products such as methyl formate, dioxymethylene dimethoxide, trioxymethylene dimethoxide and the like, which is undesirable. JP-A-59-134789 discloses that trioxane selectivity is higher when a macroreticular cation exchange resin having a sulfonic acid group is used and the concentration of formic acid is controlled at 0.025-15.0 wt. %. The concentration of formic acid in this literature, however, refers to a formic acid concentration in raw materials mixture fed into reactor, i.e. a formic acid concentration before start of reaction and is not a formic acid concentration in reaction system during reaction. Incidentally, too high a formic acid concentration causes equipment corrosion by formic acid.